Humans have more lower-back vertebrae than chimps, and that lets our spines curve
Between your ribs and pelvis sit five heavy vertebrae, the largest bones in the spinal column. Chimpanzees and gorillas have only three or four, which leaves them unable to arch the lower back. Fossils now suggest our shared ancestor had a long back, and the great apes shortened theirs independently.
The lumbar vertebrae, labelled L1 to L5 from the top, carry much of the body's weight while still allowing movement. Each has a kidney-shaped body, wider side to side than front to back, and an arch that encloses a triangular canal and supports seven bony projections. You can tell them apart from neck and chest vertebrae by what they lack: no holes in the side projections, as neck vertebrae have, and no joint surfaces for ribs, as chest vertebrae have.
Details shift down the column. The pedicles, the sturdy struts joining body to arch, widen from about 9 millimetres to as much as 18 at L5 and tilt from 10 to 20 degrees, and surgeons sometimes use them as an entry point for fixation screws. The first lumbar vertebra lines up with the transpyloric plane, a landmark level with the outlet of the stomach, the kidneys' blood vessels and the end of the spinal cord. The fifth has a wedge-shaped body, deeper in front, and it is by far the most common site of spondylolysis, a defect of the vertebral arch.
Not everyone has five. Some people have four or six, so problems that usually strike L5 turn up at L4 or L6 instead. Among four-legged animals the count varies too, six in horses and cattle and six or seven in pigs, and in butchery that stretch of spine yields tenderloin and sirloin.
The evolutionary story rests on Nacholapithecus, a tailless Miocene ape with six lumbar vertebrae whose lower back resembles those of early Australopithecus and early Homo. Along with Old World monkeys and Proconsul, it could curve its lower spine, suggesting the last common ancestor was not short-backed after all.
Source: Lumbar vertebrae
